Raman chemical imaging combines Raman spectroscopy and digital imaging for the molecular-specific analysis of materials. Raman chemical imaging has traditionally been performed in laboratory settings using research-grade light microscope technology as the image gathering platform. However, Raman chemical imaging is applicable to in situ industrial process monitoring and in vivo clinical analysis. The application of chemical imaging outside the research laboratory has been limited by the lack of availability of stable imaging platforms that are compatible with the physical demands of industrial process monitoring and clinical environments. Both industrial and clinical settings often require compact, lightweight instrumentation suitable for the examination of remote areas that are inaccessible to conventional Raman instrumentation and involve harsh chemicals in hostile environments.
Raman spectroscopy is an analytical technique that is broadly applicable. Among its many desirable characteristics, Raman spectroscopy is compatible with samples in aqueous environments and can be performed on samples undergoing little or no sample preparation. The technique is particularly attractive for remote analysis via the use of optical fibers. By employing optical fibers as light delivery and collection, the light source and light detector can be physically separated from the sample. This remote attribute is particularly valuable in sensing and analysis of samples found in industrial process environments and living subjects.
In a typical fiber-optic-based Raman analysis configuration, one or more illumination fiber-optics deliver light from a light source (typically a laser) through a laser bandpass optical filter and onto a sample. The laser bandpass filter allows only the laser wavelength to pass while rejecting all other wavelengths. This purpose of the bandpass filter is to eliminate undesired wavelengths of light from reaching the sample. Upon interaction with the sample, much of the laser light is scattered at the same wavelength as the laser. However, a small portion of the scattered light (1 in 1 million scattered photons) is scattered at wavelengths different from the laser wavelength. This phenomenon is known as Raman scattering. The collective wavelengths generated from Raman scattering from a sample are unique to the chemistry of that sample. The unique wavelengths provide a fingerprint for the material and are graphically represented in the form of a spectrum. The Raman scattered light generated by the laser/sample interaction is then gathered using collection optics which then direct the light through a laser rejection filter which eliminates the laser light, allowing only Raman light to be transmitted. The transmitted light is then coupled to a detection system via one or more collection fiber-optics.
Previously described Raman fiber optic probe devices have several limitations. First, current fiber-optic-based Raman probes are sensitive to environmental variability. These devices often fail to function properly when the probe is subjected to hot, humid-and/or corrosive environments. Several fundamental differences from current devices have been incorporated into the chemical imaging fiberscope design described here that address the environmental sensitivity issue. First, an outer jacket that is mechanically rugged and resistant to high temperatures and high humidity has been incorporated into the fiberscope design. Second, an optically transparent window that withstands harsh operating environments, has been built into the probe at the fiberscope/sample interface. Normally, incorporation of a window into a probe would introduce a significant engineering problem. As emitted illumination light passes through the window and onto the sample, a portion of this light is back reflected by the window's inner and outer surfaces. In the prior art, this undesired back reflected light is inadvertently introduced into the collection fibers along with the desired Raman scattered light. The back reflected light corrupts the quality of the analysis. This problem is addressed in the current design by careful engineering of the aperture of the collection bundle taking into account the numerical apertures (NA) associated with the collection bundle fibers and collection lenses.
Previous probe designs are also inadequate because of the environmental sensitivity of the spectral filters that are employed in the devices. The Raman chemical imaging fiberscope design of the current invention relies on spectral filter technologies that are remarkably immune to temperature and humidity. Past spectral filters have traditionally been fabricated using conventional thin film dielectric filter technology which are susceptible to temperature and humidity induced degradation in the filter spectral performance. The spectral filters described in the present invention employ highly uniform, metal oxide thin film coating materials such as SiO2, which exhibits a temperature dependent spectral bandshift coefficient an order of magnitude less than conventional filter materials. The improved quality and temperature drift performance of metal oxide filters imparts dramatically improved environmental stability and improved Raman performance under extreme conditions of temperature and humidity.
A final limitation of current probe technologies is that none combine the three basic functions of the chemical imaging fiberscope: (1) video inspection; (2) Raman spectral analysis; and (3) Raman chemical image analysis, in an integrated, compact device.
Raman chemical imaging integrates the molecular analysis capabilities of Raman spectroscopy with image acquisition through the use of electronically tunable imaging spectrometers. Several imaging spectrometers have been employed for Raman chemical imaging, including acousto-optical tunable filters (AOTFs) and liquid crystal tunable filters (LCTFs). For Raman imaging, LCTFs are clearly the instrument of choice based on the following demonstrated figures of merit: spatial resolving power (250 nm); spectral resolving power (<0.1 cm−1); large clear aperture (20 mm); and free spectral range (0–4000 cm−1). AOTFs and LCFPs are competitive technologies. AOTFs suffer from image artifacts and instability when subjected to temperature changes.
Under normal Raman imaging operation, LCTFs allow Raman images of samples to be recorded at discrete wavelengths (energies). A spectrum is generated corresponding to thousands of spatial locations at the sample surface by tuning the LCTF over a range of wavelengths and collecting images systematically. Contrast is generated in the images based on the relative amounts of Raman scatter or other optical phenomena such as luminescence that is generated by the different species located throughout the sample. Since a spectrum is generated for each pixel location, chemometric analysis tools such as Cosine Correlation Analysis (CCA), Principle Component Analysis (PCA) and Multivariate Curve Resolution (MCR) are applied to the image data to extract pertinent information.